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Caption

Influenza viruses. Coloured transmission electron micrograph (TEM) of influenza (flu) viruses. Influenza viruses belong to the orthomyxovirus group of viruses. They contain a core of RNA (ribonucleic acid) genetic material inside a protein coat or capsid (green). Protruding from the capsids are surface spikes (not seen) which adhere to the host cell. Occasionally the antigen spikes mutate and change form. While all changes decrease the ability of the immune system to fight flu, it is only when a completely new strain of virus results that a flu epidemic may occur. To protect those at risk, new flu vaccines must be prepared every year. Magnification unknown.